High-Frequency Power Conversion for Machine Drive Applications

Dr Niall Oswald

Electrical Energy Management GroupUniversity of Bristol

niall.oswald@bristol.ac.ukhttp://www.bris.ac.uk/engineering/research/em/

IntroductionPresently working on two projects:

• EPSRC ‘Centre for Power Electronics’ components theme• Focus is on wound passive components – performance and lifetime when

subjected to high speed switching waveforms.

• TSB ‘Power Module Validation’ (PoMoVal)• Development, test and analysis of a high-power, high-frequency power module

for Electric Engine Start functionality.• Safran Power UK, Raytheon Systems Ltd, UoB.

• These projects have similar objectives – enabling compact, efficient power conversion for machine drive applications, using new device technologies.

Opportunities…Low per-cycle switching loss of new devices (WBG) allows high switching frequency operation, in turn enabling:

• High output frequency• High speed, torque-dense machines (e.g. 10 pole traction machine @ 12k rpm – 1 kHz

fundamental).• High switching frequency reduced current waveform distortion.

• Compact, lightweight output filtering• Sinusoidal output voltage, low harmonic distortion output current waveform –

reduced losses in driven machine.• Possibility of eliminating screened cables – desirable in many applications, reduces

installation cost.• Filter cut-off frequency can be placed outside controller bandwidth.

However…

…and ChallengesLow per-cycle switching loss is largely due to increased switching speed – dominant DC-AC power conversion topologies (2L-VSI, 3L-NPC) are hard-switched.

• ‘Slow’ IGBT-based drives already require significant EMC countermeasures to achieve compliance.

• SiC MOSFETs capable of switching at 10s of kV/μs.

• Neither practical nor desirable to have this level of dv/dt present at inverter output terminals – essentially mandates use of output filtering!

• Filter components must withstand switching stress and provide sufficient attenuation of increased high-frequency spectral content.

• Filter topology selection and parasitic circuit elements important in determining overall performance.

Oswald, N.; Anthony, P.; McNeill, N.; Stark, B.H., "An Experimental Investigation of the Tradeoff between Switching Losses and EMI Generation With Hard-Switched All-Si, Si-SiC, and All-SiC Device Combinations," IEEE Transactions on Power Electronics, May 2014

600 V, 10 A

+40 dB above 16 MHz

Project Plans & ObjectivesKey objectives:• Development of wound component high frequency electrical

behavioural models.• Investigation of aging effects of high dv/dt on wound

components.

Exemplary application – 40 kVA all-SiC 2L-VSI.• Consideration of application requirements provides

representative basis for filter design (cut-off frequency, voltage drop, etc.).

• Based around commercially-available 1200 V, 100 A SiC MOSFET module (CREE CAS100H12AM1).

• Single phase inductor test-bed circuit under construction.

Project Plans & Objectives• Test-bed circuit designed to maximise switching

performance (not cost/size optimised!)

• Draws on previous experience (PhD research, PoMoVal test circuits).

• Loss analysis used to determine capabilities of CREE all-SiC module.• 100 ARMS @ 40 kHz

• 50 ARMS @ 100 kHz• 30 ARMS @ 200 kHz

• Initial tests will use existing UoB-designed high-performance inductor.

Potential Outcomes & Exploitation Plans

• Improved understanding of trade-offs between total filter volume and switching frequency, subject to EMC limitations.

• Development of improved (compact, low-loss, low-EMI) output filtering topologies using models developed by this research.

• Construction of demonstration high-frequency inverter?